Foil based film heaters, for example copper foil based heaters, have been in use for various heating applications, for example, pressure sensitive surface heating applications. However, foil based thin film electrodes are prone to catastrophic failure due to puncture, fatigue and internal shorting. Reactive metal failures in foil based film heaters have also been present due to outgassing of urethane foam byproducts, resulting in rapid corrosion and degradation of load carrying conductor metal.
In light of these and other shortcomings in foil based film heaters, thin film composite and low density electrode heater designs were developed. One such design for a flexible film based heater includes a conductive carbon film substrate, such as carbon impregnated Kapton® (manufactured by E.I. du Pont de Nemours and Company), with conductive electrodes, such as printed silver electrodes, located on top of and in electrical communication with the conductive substrate. This design is useful for various applications, such as those associated with the automotive industry, for example, seat heaters, mirror heaters, and fluid reservoir (such as water tank) heaters. Electrical current flows through the electrodes and through the conductive substrate. Heat is generated as electricity flows through the substrate due to the resistance of the substrate. One particular application is, for example, a seat heater that conforms with Daimler Chrysler specification number CN 40715-V01-AC.
The conductive carbon film substrate with conductive electrodes design has demonstrated an improved resistance to thermal and mechanical degradation under general operating conditions over the foil based heaters. Other advantages realized by flexible film based heaters include, but are not limited to: reduced weight, lower electrical power requirements, and improved thermal transmission properties. Additionally, film based heaters tend to resist perforations, resist cracking and withstand a large number of puncture sites without creating a thermal incident failure of the thin film structure, and flexible film based heaters exhibit these characteristics over a wide range of operating conditions. As an example, when a film based heater is perforated, the perforation results in reduced thermal energy output, with the output reduction being proportional to the extent of puncture. Generally, as compared to foil based heaters, film based heaters are intrinsically safer and not as prone to catastrophic failure even when used in active flexing or extreme temperature applications. Film based heaters are typically compatible with dissimilar metal configurations, situations involving chemical out-gassing or perforation degradation, and do not typically result in non-conforming thermal incidents.
Known manufacturing methods for film based heaters have been limited to slow, non-automated processes. One reason for this limitation was the belief that automated processes could not apply a sufficiently thick layer of conductive ink to the substrate to generate an operable heater electrode. One known method for manufacturing the film based heaters is a silk screen transfer printing process that deposits a silver ink onto a conductive substrate, such as carbon impregnated Kapton®. The ink is forced by a rubber blade, generally handheld, through a mesh stencil, or screen, onto an individual piece of substrate. Once a piece of substrate is printed and removed from the printing press, another piece of substrate is inserted into the printing press to have ink applied. Following printing, each piece of printed substrate is individually sent through a drier to complete the process. The silk screen method is capable of achieving minimum silver ink thickness requirements and was the method recommended by engineers of the company producing the carbon impregnated Kapton® substrate that was used in the known silk screen manufacturing method as the only acceptable method of silver ink application. These engineers further considered the use of highly automated transfer printing of silver ink deposits on carbon impregnated sheeting as being generally impossible.
The silk screen technique, however, has a number of shortcomings. For example, the silk screen method is too slow to be commercially viable, especially for many large scale production purposes. As another example, the silk screen method is prone to inconsistencies between applications due to differences in how the ink spreads from one application to another.
Consequently, there is a need for an improved manufacturing method and apparatus for a film based heater. Certain preferred features of the present invention address these and other needs, and provide other important advantages. Some or all of these features may be present in the corresponding independent or dependent claims, but should not be construed to be a limitation unless expressly recited in a particular claim.